The proposed research examines the early development of physiological and behavioral reactivity to emotionally-salient events during infancy. Convergent methodologies from studies of fear-potentiation of startle in animals and studies of affective modulation of blink reflex responses in human adults are adapted in order to investigate the emergence of infants' sensitivity to affective information conveyed by emotional expression. The methods employ a sudden brief acoustic noiseburst as a physiological probe" of the infant's affective state during the viewing of emotionally expressive faces. Like adults, 5-month-old infants show potentiated reflex responses during negative, relative to positive and neutral, affective displays. The proposed research incorporates three areas of inquiry. The first cross-sectional project delineates the development of affective reflex modulation during early infancy. Developmental comparison of changes in reflex size and speed with changes in overt behavioral responses will determine whether physiological responsivity to affective valence precedes or coincides with behavioral reactivity. The second objective is to discover whether infants, like adults, demonstrate right-hemisphere lateralization of affective signal processing. The startle blink paradigm is well-suited for the study of laterality effects because it allows comparison of how responses to left-ear and right-ear acoustic probes are influenced by the presentation of affective expressions. The final project assesses individual differences in sensitivity to affective modulation. Infants who are highly reactive to unfamiliar sensory events may also show greater potentiation of reflex activity during negative affective displays. This finding would link stable individual differences in behavioral inhibition, demonstrated in human infants and children, with an anatomically distinct pathway for fear modulation, demonstrated in animals. The proposed research is expected to clarify mechanisms involved in the perception of emotional signals and the regulation of emotional responses; both are integral processes in early psychological development.